In a crystal-based solar cell for use in space, a cover glass using borosilicate glass as a base material and having a high infrared emissivity is attached thereonto. Based on the high emissivity, the cover glass serves as means for releasing heat from a surface of the glass to outer space. In space applications of a thin-film solar cell which has been originally developed for use on the ground, if the conventional space cover glass is used therein, advantages of the thin-film solar cell will be spoiled in terms of weight and flexibility. Differently from the crystal-based solar cell, most of the thin-film solar cells have a top layer formed of a transparent conductive film with a low infrared emissivity. Therefore, if such a thin-film solar cell is used in space without any measures, it will have a higher temperature than usual due to insufficient heat release to cause deterioration in cell efficiency. While there is a technique of enclosing an on-ground solar cell with an organic material, mechanical/optical characteristics of the organic material are highly likely to deteriorate in space due to harsh radiation environment. Thus, a new heat control technique as an alternative to the cover glass is essentially required to use a thin-film solar cell in an adequate temperature range in space environments.
As a technique for shielding unnecessary solar radiation, there has been known a cover glass applied with a Blue Red Reflector (BRR) coating. This cover glass is formed with several dozen layers of an optical thin film on each of front and rear surfaces thereof to reflect ultraviolet rays and near-infrared lights so as to prevent excess thermal input and achieve lower temperature of a solar cell in space environments. Even though this cover glass is excellent in performance, it involves problems about its complicated forming process and the need for optimizing film-forming parameters of the optical film, such as a substrate temperature (a temperature of a substrate of the solar cell), in consideration of a thin-film solar cell. Thus, it is required to develop an optical film capable of being produced in a simplified manner through a film-forming process suitable for a thin-film solar cell.
[Non-Patent Publication 1] C. Kitchen, K. Mullaney, M. Price, A. Dollery, K. Fyles, H. Eaves, R. Crabb and P. Buia, “Solar cell coverglass for satellites in the intermediate earth orbit”, Conference record of the 26th IEEE Photovoltaic Specialists Conference (1997) 1011-1014
[Non-Patent Publication 2] A. Ohnishi, T. Onoda, S. Yamamoto, S. Sanbe, Y Morita, “Development of a New Borosilicate Cover Glass and a Second Surface Mirror for Spacecrafts” Trans. IEE of Japan Vol. 115-A No. 6, (1995) pp 471 to 478
[Non-Patent Publication 3] J. R. Tuttle, A. Szalaj and J. Keane, “A 15. 2% AM0/1433 W/kg thin-film Cu(In, Ga)Se2 Solar cell for space applications”, Proceedings of 28th IEEE Photovoltaic Specialists Conference (2000)
[Non-Patent Publication 4] M. Kroon, G. Oomen, and R. van der Heijden, “End-of-Life power predictions of Cu(In, Ga)Se2 solar cell”, Proceedings of 3rd World Conference on Photovoltaic Energy Conversion (2003) 3P-C3-55